1. Field of the Invention
The present invention relates to methods and compositions for inhibiting the formation of naphthenate precipitates or naphthenate-stabilized emulsions during contact between crude oil containing naphthenic acid and water containing cations.
2. Description of the Related Art
Crude oil includes various high molecular weight hydrocarbon-containing compounds, frequently including naphthenic acid. Naphthenic acids are classified as carboxylic acids of the general formula R—COOH, where R represents a cyclo-aliphatic structure typically having 10 to 50 carbon atoms and 0 to 6 rings. The term “naphthenic acid” is generally used to account for carboxylic acids present in crude oil, including acyclic and aromatic acids. Naphthenic acids are found predominantly in immature biodegraded crude oils. The amount of organic acid present in crude oil is expressed by the total acid number (TAN), often in units of milligrams KOH per gram of crude. For example, crude oil produced from the North Sea, the Far East and Western Africa exhibit high TAN numbers, such as a TAN greater than one (1), including high concentrations of naphthenic acids.
One problem experienced during production of crude oils having a high TAN number, is the formation of calcium naphthenate precipitates and/or naphthenate stabilized emulsions. The carboxylic group present in naphthenic acids exhibits a hydrophilic nature that causes the carboxyl to congregate at the oil-water interface. As the crude oil is produced, the pressure in the crude oil drops and carbon dioxide is lost from the solution. The loss of carbon dioxide causes the pH of the crude oil to increase and, in turn, leads to dissociation of the naphthenic acid. The naphthenates can then act as natural surfactants leading either to stabilized emulsions or solid deposits following complexation with calcium or other cations present in the aqueous phase. The naphthenate deposits can accumulate in flow-lines, heat-exchangers, oil-water separators, desalters, filters, hydro-cyclones, and the like.
When naphthenic acids in crude oil are combined with high pH, highly mineralized connate waters, the processes that separate oil and water can experience severe problems. The neutralization products of naphthenic acids with basic ions in the water tend to form very stable water-in-oil emulsions and/or insoluble sticky calcium naphthenate deposits. These phenomena can hamper the oil production considerably and result in high treatment costs.
Existing treatments to prevent the formation of calcium naphthenate precipitates and emulsions during oil production include the injection of large volumes of acetic acid in order to decrease the pH of the oil and water phases, thus favoring naphthenic acid over the formation of naphthenate salts. However, to accomplish this task requires continuous injection of about 100 to 1000 parts per million (ppm) of acetic acid to achieve a pH below at least 6.0 and sometimes even below 5.2. However, this acid is very corrosive and presents various health, safety, and environmental issues. Furthermore, the shear volume of acetic acid solution that is necessary represents a significant storage and supply problem, especially on offshore oil production platforms. Also, lowering the pH of the produced waters may lead to serious internal corrosion of the crude oil processing equipment. The result is that preventing the formation of naphthenate precipitates and emulsions comes at a high cost.
Therefore, there is a need for an improved method of inhibiting the formation of naphthenate precipitates and emulsions during the production of crude oil. There is also a need for a composition or formulation that is more effective at inhibiting naphthenate precipitates and emulsions. It would be desirable if the method and composition required smaller volumes of active chemical, lower dosage rates, smaller pH adjustments, and lower costs.